FPSF in zone 5 with expansive soil
Hello all,
I have a foundation question. I’m in Colorado Springs, zone 5. Doing a single family build “pretty good house”. We’ve seen some articles on frost protected shallow foundations as a good choice for minimizing concrete and being suitable for expansive soil, as well as for our goal of fully insulating our slab. We have expansive clay on our site, but a reasonable slope (20″ in the 40′ of the 20×40 bldg footprint) to shed water away with drain to daylight. Local engineers are wanting to do a large overdig to replace the expansive soil and deeper footers (40-48″), rather than FPSF. It feels somewhat a case of this is what they usually do and less that FPSF is or isn’t appropriate. Would appreciate some feedback. Thanks!
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Do you really want to bet half a million dollars that you are right and the engineers are wrong?
If you want to cut your carbon foot print I say, find another option or more stable ground.
Walta
I could choose to just accept common practice (like studs 16" oc, a gas furnace, that houses leak enough to have decent air quality) or I can seek more knowledge and make an informed decision. I choose the latter which is why I ask questions on a building science site.
Do what the engineers say; don't compromise on your foundation. The carbon footprint of concrete is massively overblown in the first place.
I strongly disagree that the impact of a foundation on global warming is overblown in any way, but even I put building durability ahead of reducing environmental impact, when reasonable alternatives don't exist.
Most engineers are not familiar with FSPFs and if there's one thing virtually all engineers dislike, it's something they aren't familiar with. They are staking their insurance and license on providing solutions that have been shown to be safe for a long period.
There are exceptions; for the slabless slab I designing in 2017, I asked my most creative and environmentally-oriented PE to review my design after the CEO required it.
You can share ASCE 32-01 with the engineers; that's the source document that building codes and the HUD handbook on FPSFs use as a reference, but it goes deep into the engineering that shows safe assemblies.
All that said, I'm not sure that FSPFs are a good match for expansive soils. We wanted to do one on a current project but the geotechnical engineer and structural engineer required a full stem/frost wall.
Thanks Mike, that's another good data point and exactly the kind of info I was looking for. Mostly seems that the use in the US is still relatively low so it's a higher risk choice (which therefore for now I won't take, given that structural stability is my priority as well).
A rubble trench allows you to go to the depth you need without all the carbon you don't want.
https://www.finehomebuilding.com/1984/01/01/rubble-trench-foundations
fourforhome,
Rubble trench foundations are specifically not recommended for expansive soils.
Pier and grade beam would probably entail less concrete, but you wouldn’t have a slab anymore….
There are a lot of benefits to having a crawlspace though, especially where utilities are concerned.
Do a Google search for "raft slab foundation expansive soils" and you will find several useful results. This type of foundation uses more concrete than a shallow slab on grade, but probably less than what your engineer is suggesting. Raft slabs are a very standard solution and I would be very surprised if your engineer isn't aware of them. Perhaps there is some particular reason for why they aren't suitable for your case. But it would be worth asking about them.
I'll check that out, thanks